The field of the invention is that of echographic imaging. The invention relates to methods and devices for imaging and treating pathologies of human organs.
It relates more particularly to methods and devices for imaging with high spatial and temporal resolution based on the use of synthetic methods. The temporal resolution increases as the number of firings necessary to reconstruct the image decreases. The image acquisition rate, i.e. the number of images per unit time, is directly related to this temporal resolution.
Standard echographic imaging systems utilize the echoes backscattered by the medium to be probed, generally a biological tissue, to analyze the variations in acoustic impedance characteristic of biological structures and thus to reconstruct an image of that medium.
An ultrasound image is typically obtained by generating and transmitting beams focused at a given focal distance and transmitted in a given direction to produce what is referred to as a line of the image. This is shown in FIG. 1, in which a delay law LR diagrammatically represented by a dashed line curve is applied to an array of transducers T1 to TN. This generates a beam B focused around a point F. Transverse scanning, diagrammatically represented by an arrow, is effected over the length of the array of transducers. The corresponding line of the image is then reconstructed by focusing the received signals. The whole image is obtained by transverse scanning of the area of interest using successively offset imaging lines. An optimum imaging area ZIO is then observed.
This imaging method generally uses matrix, linear, or curved echographic probes comprising a plurality of transducers, for example piezoelectric elements, used for transmission and reception. These transducers are controlled individually via independent electronic channels capable of applying to them electrical signals delayed relative to one another. Transmission focusing is effected by applying delays to the various signals transmitted. These delays correspond to the time of flight differences between the various antenna elements and the focal point, thus creating the acoustic equivalent of a lens.
Thereafter, dynamic focusing laws, i.e. a delay law for each reconstructed pixel, are used on reception to isolate the acoustic signatures coming from a given location of the medium and reconstituting its acoustic image. This is known as beamforming.
This method, which is very widely used in commercial systems, is called the mode B method. Image quality is optimum for depths close to the focal distance but is degraded on moving away from the focal spot.
The number of characteristic firings to produce such an image is generally equal to the number of reconstructed lines and is of the same order of magnitude as the number of antenna elements, typically 128 or 256.
Variants of this method have been developed.
The depth multi-focus method consists in determining a plurality of focal distances and reconstructing the line portions situated in the vicinity of the various focal points. This method improves image quality but increases the number of firings necessary by a factor Nfoc that is the number of focal distances used. This is shown in FIG. 2 and described in U.S. Pat. No. 5,113,706. Successive delay laws LR1 to LR4 are transmitted, each generating a beam focused at a different point F1 to F4. It is seen that a wider optimum imaging area is obtained.
The synthetic transmit aperture method consists in transmitting unfocused beams emanating successively from each of the elements of the antenna and then reconstructing for each of the firings a so-called “low resolution” image by reception focusing. This is shown in FIG. 3. In FIG. 3A, a first antenna element transmits a wave towards a diffusing medium M. The signal is diffused and reflected by the medium. Then, in FIG. 3B, a second antenna element T2 transmits the same wave toward the medium M, and so on for all of the antenna elements T1 to TN.
The data set acquired after transmission from each of the antenna elements T1 to TN in succession is called the complete data set. The final image is obtained by summing the partial images coherently in amplitude and in phase, which images are referred to as “low resolution” images. In contrast to a standard imaging mode, an image is obtained with dynamic transmission focusing, which focusing is synthetic. It is for this reason that the term synthetic transmit aperture is used. The image obtained in this way is of optimum quality and the number of firings necessary is equal to the number of antenna elements.
The above method has the major drawback of not enabling areas that are too far from the antenna to be imaged. The ratio between the signal and the thermal noise caused by the sensors is lower than for the standard method by a factor √Nel, Nel being the number of the elements of the antenna. This is because, for imaging the same pixel, the standard method requires only one acquisition, whereas the coherent synthesis method requires the acquisition of Nel firings, that is to say, for Gaussian white noise, √Nel times more noise. This is described in U.S. Pat. No. 5,623,928 and in U.S. Pat. No. 4,604,697.
To alleviate the problem of the signal-to-noise ratio of synthetic aperture imaging, a so-called spatial coding approach has been developed. This is based on defining and using a transmission matrix. The transmission matrix is defined by concatenating the various weighting laws of the antenna during successive transmissions.
In the special case of acquiring the complete data set, the transmission matrix used is the identity matrix.
The spatial coding method consists in sounding the medium with the weighting laws contained in the transmission matrix, chosen beforehand to be reversible. This is shown in FIG. 4 where it is seen that each of the transducers T1 to TN transmits with a different but predefined intensity. The intensities for each firing constitute a vector of a transmission matrix ME grouping the successive intensities at each transducer. The signals acquired in this way are then projected into the so-called canonic base, i.e. each matrix composed of signals received by the transducers at a given time during the transmission-acquisition process is leftward multiplied by the inverse of the transmission matrix.
This technique enables the complete data set to be acquired from a transmission matrix ME that is different from the identity matrix. To be more precise, any transmission matrix may be used on condition that it may be inverted.
The major benefit of this technique is that it enables improvement of the signal-to-noise ratio of the synthetic aperture imaging method by a factor equal to the determinant of the transmission matrix.
This method, initially introduced by Chiao, notably in U.S. Pat. No. 6,048,315, in the context of medical ultrasound, as mainly used with Hadamard transmission matrices. These are easier to implement and they make optimum signal-to-noise ratios possible.
The aperture synthesis and incoherent summing methods are sometimes used simultaneously, for example as in document US 2003/0149257.
A synthesis method that is not based on the transmission matrix consists in coherent summing of images formed from transmissions of unfocused depointed waves. Here a delay law is applied such that the wave front is at a predetermined angle to the surface of the probe. In this way, the transmitted wave propagates in a direction at a particular angle to the normal to the probe. This method offers the same performance as spatial coding and is described in document US 2003/0125628. There it is a question of synthesizing dynamic focusing on transmission by transmitting unfocused waves at different angles. That technique is close to the aperture synthesis method described above, with the difference that unfocused waves are transmitted instead of circular waves.
A number of methods have been developed in recent years, most often based on standard mode B imaging methods and aiming to augment the image acquisition rate.
The multi-line method, shown in FIG. 5A, consists in widening the transmission beam B using a particular transmission law LRE different from the particular reception laws LRR1 and LRR4 and adapted to enable the reconstruction of a plurality of Nline lines in parallel (here four lines in parallel). The image acquisition rate is multiplied by Nline but image quality in terms of resolution and contrast is degraded. This is described in the document by D. P. Shattuck et al. “Explososcan—a Parallel Processing Technique for High-Speed Ultrasound Imaging with Linear Phased-Arrays”, Journal of the Acoustical Society of America, vol. 75, pp. 1273-1282, 1984. An optimum imaging area ZIO similar to that of the mode B method is obtained.
The multi-beam method shown in FIG. 5B consists in simultaneously transmitting a plurality of Nbeam beams B1 to B3 each focused at a point F1 to F3 using simultaneous transmission laws LR1 to LR3 and reconstructing a plurality of lines simultaneously. That method reduces the number of firings by a factor Nbeam but degrades image quality. That method is known from the thesis of J. Bercoff, “L′imagerie échographique ultrarapide et son application à l′étude de la viscoélasticité du corps humain” [Ultrafast echographic imaging and application to studying the viscoelasticity of the human body], Paris 7, 2004. The optimum imaging area ZIO obtained is similar to that obtained with the mode B method.
For its part, the unfocused wave mode, shown in FIG. 5C, consists in transmitting an unfocused wave OP and in reconstructing all of the lines of the imaged area ZI simultaneously. The unfocused wave may be a plane wave generated with no phase shifting applied to transmission by the various antenna elements T1 to TN. That method, which is optimum in image acquisition rate, exhibits strongly degraded image quality.
With the growth of 3D imaging systems, a so-called adaptive image acquisition rate imaging method has recently been developed. That method consists in taking into account the imaging context to adapt the image acquisition rate and consequently adapt image quality.
U.S. Pat. No. 6,346,079 discloses estimating the movement of the medium to be imaged and adjusting the number of firings accordingly. The movement is estimated by measuring the correlation of the brightness of successive images or by Doppler analysis of the acquired signals. The imaging method is of the mode B type. The number of firings necessary is varied by varying the aperture of the transmitted beams and thus reducing the number of imaged lines. Once again, the image acquisition rate is improved to the detriment of image quality.
As already seen, synthetic aperture methods furnish a set of complex so-called “low resolution” images. It is possible to weight the coherent summing in various ways as a function of what is required.
Thus spatial weighting may be effected. That consists in weighting the pixels of the low-resolution images as a function of the position of the pixel relative to the transmitter. If it is in the main transmission lobe, it is given a maximum weighting, whereas if it is outside that lobe its weighting is close to zero. That weighting enables the quality of the images to be greatly increased. Conventionally used cardinal sine, Tchebychev, or Hanning type weighting gives good results but the number of firings necessary is not reduced.
Weighting may also be effected by coherence measurement. Statistical measurements are then effected on the low-resolution sets of pixels, notably coherence measurements. Since anechoic areas are theoretically incoherent (white noise), weighting by the coherence map is going to lead to an increase in contrast. The idea is to use the coherence measurement of the same pixel between the so-called low-resolution images to improve the quality of the final so-called high-resolution image. That approach may be accentuated by weighting with the exponential of the coherence, but that leads to an increase in the sharpness of speckle. Finally, the use of a pre-adjusted error function enables contrast to be increased without degrading speckle. Nevertheless, it is again not possible to improve the image acquisition rate and that technique may be applied only to synthetic aperture systems.
The constant improvement in computation power and the increasing integration of programmable electronics of the field programmable gate array (FPGA) type are changing the nature of the problem of ultrasound imaging image acquisition rates. The image acquisition rate is less and less limited by the image reconstruction time but rather by the flight time of the beams or, in other words, the number of firings necessary to reconstruct the final image.
In parallel with this, the requirements for imaging with a high temporal resolution are of three kinds:                Echography of the heart, where improving the temporal resolution would enable valve pathologies to be detected;        Elastography, where it is necessary to visualize the propagation of shear waves in tissues with high temporal resolution;        3D imaging, where standard focused transmission methods are unable to achieve high temporal resolutions.        
Improving temporal resolution usually consists in widening the transmitted beams to enable the number of firings to be reduced. This reduction has the effect of degrading image quality in terms of resolution and contrast.